The present invention relates to an improvement in the Investment Casting process. The Investment Casting Process or "Lost Wax" Process has been practiced for centuries. The original process consisted of forming an original wax model of the part or casting which was to be made. The wax model was coated with a slurry of plaster. Successive layers of plaster were added to the was model until a strong shell enveloped the model. A variation in the process involved dipping or "investing" the wax model into a bucket full of plaster slurry. The plaster covered model was then heated until the wax melted out of the plaster shell, thereby leaving a void which had the shape of the part which was to be made. The void was then filled with molten metal. The result was a casting was a perfect duplication of the original wax model.
The Investment Casting Process is being used extensively today as a shortcut to producing finished high quality precision parts which, otherwise, would have to be made by precision machining. Complex parts with undercut and smooth surfaces, accurate dimensions and fine details can be made by investment casting. In accordance with present day procedures, a wax pattern is made by injecting hot wax into a metal mold. The pattern has the exact geometry of the desired finished part. The pattern is slightly larger than the finished part to compensate for volumetric shrinkage of the hot wax model and metal casting during solidification. The shell is formed by dipping the wax pattern into a ceramic slurry and then coating or "stuccoing" it with fine granular ceramic material, allowed to air dry and then repeating the operation several times until a shell is formed around the pattern. In order to increase production, several wax patterns are attached to a central wax sprue and coated with ceramic material as a cluster. Each pattern has a gate which enables the pattern to be attached to the sprue and to provide a passage for drawing out the pattern material as it melts. The gates and the sprue also guide the molten metal to the voids in the ceramic shell during the casting operation.
In spite the many advantages of investment casting over other procedures, the investment casting process is still relatively time consuming and expensive. Every step of the investment casting process requires highly skilled operators and great care in carrying out the steps of the operation. Great care has to be exercised in gluing all wax patterns to the gates and the gates to the sprue, also that the wax stays at constant temperature to avoid expansion of the wax which would crack the ceramic shell. A crack in one of the layers of the shell can cause the entire shell to fracture during the steam autoclave, firing or casting steps of the process. Shell breakage affects productivity and adds substantially to the cost of the operation. Most of the damage to the shell occurs during the step of investing melting the wax in a steam autoclave. Since wax expands when heated, it must be evacuated from the ceramic shell as quickly as possible. Otherwise, the expanding wax creates an internal pressure within the shell and either breaks the shell or creates a fracture which causes the shell to break during the casting step. The wax from the melted pattern is usually not recycled to make patterns. In order to maintain the high quality which is required of investment castings, new wax must be used each time a new wax pattern is made, thereby adding considerably to the cost of the operation. These and other difficulties have been obviated by the present invention.
It is, therefore, a principal object of the invention to provide an investment casting process in which a model or pattern of the item to be made is made of a material which is liquid at room temperature and which does not expand within the shell during the step of shell building and of melting and evacuation of the model from the ceramic shell.
Another object of this invention is the provision of an investment casting process in which a cluster of models or patterns is formed in a single molding step, including pouring cup, sprue and gates.
A further object of the present invention is the provision of an investment casting process in which the model which is to be invested is made from an inexpensive recyclable material which does not expand when changing from the solid state to the molten state.
With these and other objects in view as will be apparent to those skilled in the art, the invention resides in the combination of steps set forth in the specification and covered by the claims appended hereto.